Retroviral vectors provide the most efficient means of stably expressing genes in cells both in vitro and in vivo. We are interested in improving the efficiency of retroviral-mediated gene transfer into cells that are refractory to infection by the currently available retroviral vector systems. As a first step towards improving the efficiency of retroviral gene transfer we are characterizing the cellular and retroviral factors that inhibit efficient infection of these cells. We have determined that some cells can be relatively resistant to infection due to inefficient interaction of the retrovirus with the type of receptor expressed on the surface of the target cell (e.g. Moloney murine leukemia virus infection of the murine cell line, MDTF). Other cells resist infection due to blocks effected at a post-receptor-binding stage of viral infection (e.g. infection of human myeloid cells by murine-based retroviral vectors, infection of hamster cells by GaLV, gibbon ape leukemia virus). From these analyses it has become apparent that it is not feasible to develop a universal vector capable of infecting all types of cells with an equal efficiency. Rather, for specific target cells it may be necessary to customize a retroviral vector for optimal gene delivery. We are developing a GaLV-based retroviral vector system that should prove more efficient than the murine vector systems for introducing genes into human cells.